I submitted by math paper to an Elsevier journal around 6 months ago. Today I logged into EES and saw that the status changed to "Required Reviews Completed" about 10 days ago. Is this good news or bad news ? Does it mean my paper is rejected and the Editor is basically preparing the rejection letter :( ?
You need to learn to have patience with the peer review process. It means neither good nor bad, just that the reviews are in. You can't see further into this hole than that.
It doesn't even necessarily mean that all reviews are completed. Sometimes editors invite more reviewers than is required. This message only means that the configured lower bound has been reached.
This simply means the reviews are completed. It is not good news or bad news, it simply means the reviews are available to the editor. Hopefully you hear back soon, although frankly sometimes even at this stage it can take weeks (even months)!
I had the same question and I got in touch with the journal editorial office and got a quick reply saying:
Upon checking, I noticed that some reviewers have yet to submit their reviews. The status "Required Reviews Completed" shows when the required number of reviews is reached. Please note that the editor is still waiting for additional reports for your manuscript. Rest assured that you will be notified once updates are available.
So as others mentioned above, some reviews came through, maybe 2 as the minimum default on the system, and there still some to come so the editor is waiting to receive all prior to submitting a final verdict.
It means just what it says on the tin - that the required reviews are complete. It says nothing about their content, and whether a paper is accepted or not, it will likely come with a number of revisions and the like from reviewers. The editor needs to read the reviews, decide whether or not to accept the manuscript (and there are several different scales between just accept and reject), compile those comments into something coherent, etc.
The only thing you can divine from that message is that it wasn't rejected without review, which is also a potential outcome of a paper submission.
Based on my experience, it is a good news that you will be hearing soon from the editor regarding your paper (although you might hear a bad news).
In general, it is good to get an idea about the time frame for the journal your are submitting to. I use the Journal Finder from Elsevier (http://journalfinder.elsevier.com), which you can use to make a decision regarding the journal. This will show you the different times and acceptance rate.